Antagonism Between a Baculovirus and Bacillus thuringiensis against Plutella xyllostella1 Larvae at Laboratory and Field Conditions

Efficacy of entomopathogens for biological control of pests is frequently enhanced by modifying techniques applied in the field. One variation includes use of mixtures of different agents such as bacteria and viruses. In this report, mixtures of a baculovirus (PlxyNPV) and Bacillus thuringiensis Berliner, 1915, strain were tested by analyzing their joint-action effectiveness. Because four strains of B. thuringiensis previously showed important amounts of toxin against diamondback moth larvae, Plutella xylostella Linnaeus, 1758, an initial study selected the most toxic among the four strains. LC50s were compared, and strain LBIT-229 was selected (LC50 =170 ng/cm2) to be tested in mixtures with the baculovirus strain once LC50 for the virus was estimated individually (LC50 =425 ng/cm2). Bioassays with a 3:1 mixture of virus and LBIT-229, respectively, were used at laboratory conditions against P. xylostella 2nd-instar larvae. LC50 of the mixture was estimated at 508 ng/cm2, which was compared with the expected (theoretical) value calculated by the Tabashnik formula. According to fiducial limits from LC50 estimated empirically, both agents had an antagonistic effect. A second joint-action test was used, based on the TammesBakuniak graphical method, and found empirical LC50 in the area of antagonism, corroborating the previous result. Simultaneously, a series of experiments was done using the same mixture in a broccoli (Brassica oleracea Linneaus,1753) field, showing statistical differences among all treatments. Most diamondback moth larvae were killed when the virus was tested alone, followed by areas where the LBIT-229 strain was applied alone. Fewest larvae were killed when a mixture of both agents was tested. Antagonism between strains was clear in the laboratory and in the field.